Generally, an injection molded barrel can produce burrs at its parting line. Therefore, when the barrel engages with a corresponding base, the burrs fall from the parting line because the burrs rub against internal threads of the base, and then the burrs pollute optical elements in an interior of the base and create dust pollution.
Referring to FIG. 1 and FIG. 2, in response to the problems described above, a patent publication “Thread structure for an optical lens” (U.S. Pat. No. 7,565,078) disclosed another thread structure for a barrel, and the thread structure includes a plurality of normal threads 4 and a plurality of shrinking threads 5 recessed in the normal threads 4 near a parting line 6. An outer diameter of each of the normal threads 4 is larger than an outer diameter of a corresponding shrinking thread 5. A width of each of the normal threads 4 along a radial direction is larger than a width of a corresponding shrinking thread 5 along the radial direction. Because the width of the shrinking thread 5 is smaller than the width of the normal threads 4, the burrs at the parting line 6 are retracted from the normal thread 4. When the barrel is engaged with a corresponding base, the burrs located at the parting line 6 can't fall from the parting line 6 because the burrs do not rub against internal threads of the base, and then the dust pollution can be improved. However, a part of the burrs fallen from the parting line 6 can be accumulated in a difference space 51 easily because the plurality of shrinking threads 5 are recessed in the normal threads 4 and coordinated with the normal threads 4 to define the difference space 51. As a result, the burrs and dust accumulated in the difference space 51 easily can fall from the difference space 51 to create dust pollution during the repeated assembly process of the barrel. Moreover, the plurality of shrinking threads 5 can damage a part of the plurality of normal threads 4 to lower the yield of manufacturing the barrel during the parting mold process of the barrel.